The absolutely majority of the scholars are of the opinion that when one is in the actual rite of ‘salah’, it is best and preferred that the person offer his full prayer in the Arabic language, including the supplications which are made in the ‘salaah’ to the Lord Most High.
But at all times other than the prescribed rite of prayer, if one is not comfortable or does not understand the Arabic language, there is absolutely no harm in making supplications to their Lord Most Gracious in any language of their choice….for without an iota of a doubt, our Lord Most High hears and responds to every supplication of His believing and repentant slaves.